Hygiene or wiping products primarily include all kinds of dry-creped tissue paper, wet-creped paper, TAD-paper (Through Air Drying) and cellulose or pulp-wadding or all kinds of non-wovens, or combinations, laminates or mixtures thereof. Typical properties of these hygiene and wiping products include the reliability to absorb tensile stress energy, their drapability, good textile-like flexibility, properties which are frequently referred to as bulk softness, a higher surface softness and a high specific volume with a perceptible thickness. A liquid absorbency as high as possible and, depending on the application, a suitable wet and dry strength as well as an appealable visual appearance of the outer product's surface are desired. These properties, among others, allow these hygiene and wiping products to be used, for example, as cleaning wipes such as paper or non-woven wipes, windscreen cleaning wipes, industrial wipes, kitchen paper or the like; as sanitary products such as for example bathroom tissue, paper or non-woven handkerchiefs, household towels, towels and the like; as cosmetic wipes such as for example facials and as serviettes or napkins, just to mention some of the products that can be used. Furthermore, the hygiene and wiping products can be dry, moist, wet, printed or pretreated in any manner. In addition, the hygiene and wiping products may be folded, interleaved or individually placed, stacked or rolled, connected or not, in any suitable manner.
Due to the above description, the products can be used for personal and household use as well as commercial and industrial use. They are adapted to absorb fluids, remove dust, for decorative purposes, for wrapping or even just as supporting material, as is common for example in medical practices or in hospitals.
If tissue paper is to be made out of pulp, the process essentially comprises a forming that includes a box and a forming wire portion, and a drying portion (either through air drying or conventional drying on a yankee cylinder). The production process also usually includes the crepe process essential for tissues and, finally, typically a monitoring and winding area.
Paper can be formed by placing the fibers, in an oriented or random manner, on one or between two continuously revolving wires of a paper making machine while simultaneously removing the main quantity of water of dilution until dry-solids contents of usually between 12 and 35% are obtained.
Drying the formed primary fibrous web occurs in one or more steps by mechanical and thermal means until a final dry-solids content of usually about 93 to 97% has been reached. In case of tissue making, this stage is followed by the crepe process which crucially influences the properties of the finished tissue product in conventional processes. The conventional dry crepe process involves creping on a usually 4.0 to 6.5 m diameter drying cylinder, the so-called yankee cylinder, by means of a crepe doctor with the aforementioned final dry-solids content of the raw tissue paper. Wet creping can be used as well, if lower demands are made of the tissue quality. The creped, finally dry raw tissue paper, the so-called base tissue, is then available for further processing into the paper product for a tissue paper product.
Instead of the conventional tissue making process described above, the use of a modified technique is possible in which an improvement in specific volume is achieved by a special kind of drying which leads to an improvement in the bulk softness of the tissue paper. This process, which exists in a variety of subtypes, is termed the TAD (Through Air Drying) technique. It is characterized by the fact that the “primary” fibrous web that leaves the forming and sheet making stage is pre-dried to a dry-solids content of about 80% before final contact drying on the yankee cylinder by blowing hot air through the fibrous web. The fibrous web is supported by an air-permeable wire or belt or TAD-fabric and during its transport is guided over the surface of an air-permeable rotating cylinder drum, the so-called TAD-cylinder. Structuring the supporting wire or belt makes it possible to produce any pattern of compressed zones broken up by deformation in the moist state, also named moulding, resulting in increased mean specific volumes and consequently leading to an increase of bulk softness without decisively decreasing the strength of the fibrous web.
To produce multi-ply tissue paper products, such as handkerchiefs, bathroom paper, towels or household towels, an intermediate step often occurs with so-called doubling in which the base tissue in the desired number of plies of a finished product is usually gathered on a common multi-ply mother reel.
The processing step from the base tissue that has already been optionally wound up in several plies to the finished tissue product occurs in processing machines (converting machines) which include operations such as unwinding the base tissue, repeated smoothing of the tissue, printing embossing, to an extent combined with full area and/or local application of adhesive to produce ply adhesion of the individual plies to be combined together as well as longitudinal cut, folding, cross cut, placement and bringing together a plurality of individual tissues and their packaging as well as bringing them together to form larger surrounding packaging or bundles. Such processing steps may also include application of substances like scents, lotions, softeners or other chemical additives. The individual paper ply webs can also be pre-embossed and then combined in a roll gap according to the embossing methods known in the art. Any embossing can lead to embossed elements all having the same height or to embossing elements having different heights. Plybonding, e.g. by mechanical or by chemical means are other well-known methods mainly used for hankies, napkins and bathroom tissues.
A well-known technique to increase the thickness of a paper product is to emboss the paper web. An embossing process is carried out in the nip between an embossing roll and an anvil roll. The embossing roll can have protrusions on its circumferential surface leading to so-called embossed depressions in the paper web or it can have depressions in its circumferential surface leading to so-called embossed protrusions in the paper web.
Anvil rolls may be softer than the corresponding embossing roll and may consist of rubber, such as natural rubber, or plastic materials, paper or steel.
For manufacturing multi-ply tissue products, especially bathroom tissue and household tissue, three manufacturing methods for embossing and adhesively bonding of the plies have established. These are Goffra Incolla/spot embossing, DESL (Double Embossing Single Lamination)/Nested, and Pin-to-Pin/Foot-to-Foot.
In the first mentioned manufacturing method, Goffra Incolla, a first web is directed through the nip between an embossing roll and an anvil roll. In this nip the web is provided with an embossing pattern. Thereafter, an application roll for adhesive applies adhesive to those parts of the first web at which there are protruding embossing elements in the embossing roll. The adhesive is transported from an adhesive bath via an adhesive transfer roll to the application roll. A second web is transported to the first web and adhesively bonded to the first web in the nip between the so-called marrying roll and the embossing roll. The adhesive bonding takes place at those portions at which the adhesive was applied.
The second manufacturing method (DESL/Nested) is very similar to the above-described Goffra Incolla method. It comprises an additional pair of rolls consisting of a second embossing roll and a second anvil roll. The additional pair of rolls serves to emboss the second web before it is adhesively bonded to the first web using the marrying roll. Typically, the additional pair of rolls is placed close to the first pair of rolls and the marrying roll. Especially when using the so-called Nested-method such close arrangement is important. The Nested-method can be considered as a special case of the general DESL-manufacturing method. For the Nested-method the embossing elements of the first embossing roll and the embossing elements of the second embossing roll are arranged such that the embossed elements of the first embossed ply and the embossed elements of the second embossed ply fit into each other similar to a gearing system. This serves to achieve a mutual stabilization of the two plies. However, for the DESL manufacturing method such correlation between the embossed elements of the first, upper ply and the second, lower ply, does not have to apply. Nevertheless, in a literature the term DESL is often used synonymous to a Nested-method.
The third manufacturing method (Pin-to-Pin/Foot-to-Foot) is similar to the DESL method. By means of two pairs of rolls both the upper ply and the lower ply are embossed, respectively. Adhesive is applied onto the embossed protrusions of the first ply. The ply bonding, however, is not achieved by means of a marrying roll as in the DESL method but is achieved directly by means of the protruding embossing elements of the second embossing roll. In order to achieve this, an exact adjustment of the width of the nip between the first embossing roll and the second embossing roll is required, which is mainly defined by the individual thickness of both webs (upper ply and lower ply). Further, the embossing rolls have to be designed such that the protruding embossing elements of both rolls face each other. This is the reason why the terminology Pin-to-Pin or Foot-to-Foot embossing is used.
All above described methods have the following common features: the first embossing roll is formed of a hard material, usually metal, especially steel, but there are also known embossing rolls made of hard rubber or hard plastics materials. The embossing rolls can be a male roll having individual protrusions. Alternatively, the embossing roll can be a female roll with individual embossing depressions. Typical depths of the engraved embossing patterns are between 0.4 and 2.0 mm.
The anvil roll typically has a rubber coating with a hardness between 35 Shore A and 85 Shore A. However, structurized anvil rolls, especially rolls made of paper, rubber or plastics materials or steel are also known.
The applicator roll for adhesive is usually also a rubber roll with a plain smooth circumferential surface, wherein the hardness of the rubber coating is between the hardness of the anvil roll and the hardness of the marrying roll. Commonly used values for the hardness of the rubber coating are 70 to 85 Shore A. When selecting the rubber material its compatibility with the adhesive to be applied has to be ensured.
The application system for adhesive consisting of applicator roll, adhesive transfer roll and adhesive bath can be designed as a so-called immersion roll system in which the adhesive transfer roll is immersed into the adhesive bath and transports adhesive by means of surface tension and adhesive forces out of the adhesive bath. By adjusting the gap between the adhesive transfer roll and the applicator or application roll, the amount of adhesive to be applied can be adjusted. Application rolls may be structured rolls. Recently, adhesive transfer rolls have become known having defined pit-shaped depressions in their circumferential surface. Such adhesive transfer rolls are known as Anilox-rolls. Such roll is usually made of ceramic material or it is a roll made of steel or copper and coated with chromium. Excessive adhesive is removed from the surface of the Anilox-roll by means of a blade. The amount of adhesive is determined by the volume and the number of depressions. Alternative application systems for applying adhesives are based on a spraying equipment (Weko-technique).
A second possibility to influence the amount of adhesive transferred is the adjustment of the difference in circumferential speeds of the adhesive transfer roll and the applicator roll. Typically, the adhesive transfer roll rotates slower than the applicator roll. The circumferential speed of the adhesive transfer roll is usually between 5% and 100% of the first circumferential speed of the applicator roll. The adhesive bath can be designed as a simple trough, application systems with a blade can also be designed as chamber systems. The embossing technologies Goffra Incolla/spot embossing and DESL/Nested, both use an additional roll, the so-called marrying roll for laminating together the plies. The marrying roll commonly has a smooth rubber surface with a hardness of about 90-95 Shore A. A suitable material is e.g. NBR (acrylonitrile-butadiene rubber). However, marrying rolls also have become known which, in addition to the rubber coating, are provided with a steel coating. Such steel coating is often provided in form of a steel band spirally wound onto the rubber coating.
In case that the single layers individually or together are pre-embossed, a so-called micro-pre-embossing device is used. Such pre-embossing device is often used in combination with the Goffra Incolla technology. Also commonly used is a printing onto the tissue product before or after the ply bonding step. Also known are variants including the application of chemical substances, especially lotions and softeners.
Another well-known embossing technique comprises a steel embossing roll and a corresponding anvil steel roll (so-called Union embossing). The surfaces of these rolls are being formed in such a manner that deformation of the paper and mechanical plybonding without using adhesives are achieved within one single embossing step.
When using all of the above described three embossing methods it is advantageous to provide a control for the tension of the web both before and after the ply bonding because the physical properties of the web and especially the stress-strain characteristic can be changed significantly in the embossing step.
FIG. 11a depicts part of the circumferential surface of an embossing roll 100 having discrete protrusions 102 leading, when cooperating in the above described way with a yieldable anvil roller, to a product as shown in FIG. 11b. Such paper product 110 has a main portion lying in a base plane 112 from which embossing depressions 114 project. In the prior art example as shown in FIGS. 11a, 11b, the embossing roll has male embossing elements 102 all of which have the same height and geometry. However, a so-called double height embossing is also known in the prior art. To exemplify this, FIG. 12a shows part of the outer circumferential surface of an embossing roller 120 with two different types of male embossing protrusions. These are protrusions 122 having a higher height as well as protrusions 124 with a smaller height. Otherwise, in the example according to FIG. 12a, the protrusions have a similar cross-sectional geometry. However, it is also known in the art to combine embossing protrusions of different geometry and different height together. Such an embossing technique is known from EP 0 797 705 A1.
The resulting paper product 130 is schematically shown in FIG. 12b. Starting from a base plane 132, the paper product has embossing depressions 134 with a larger depth as well as embossing depressions 136 being smaller in size.
The embossing rolls used in the prior art are mainly manufactured using the so-called moletage technique which is a roll engraving by means of a cold forming of the steel of the embossing roll. According to such moletage technique, usually several cold forming steps and several etching steps have to be carried out. This known production technique places some limitations on the shape of the protrusions in the surface of the roll. The slope angle has an impact on the mechanical stability of the paper product produced with such an embossing roll. If the slope angle is smaller, the product has a smaller local stability around the embossed structure because locally the fiber structure is destroyed to a larger extent. Therefore, for reasons of a sufficient mechanical stability of the paper product and of the constraint put upon the manufacture of protrusions on the surface of the roll using the moletage technique, a slope angle of at least 23° is commonly used in the art.
Another engraving technology for manufacturing embossing rolls for the tissue and hygiene industry is etching. Within a first step, a mask is applied on the circumferential surface of the embossing roll followed by a second step in which the surface is etched by applying acid at those areas where no mask has been applied. The result is a circumferential surface of the roll which is partly engraved, wherein the transition between engraved areas and non-engraved areas has a step angle of 0° up to 10°. In other words, the etching technique leads to a step-like surface structure. The method steps of masking and etching with the same or different geometries of masks can be repeated several times. By using a repeated etching technique, a surface structure can be achieved which is similar to discrete stairs which extend from the original circumferential surface of the roll into the material of the roll.
A further technique for manufacturing embossing rolls for the tissue and hygiene industry is CNC milling. This technique is often used for simpler rectangular elements, for example truncated pyramids. The angle of the embossing elements and the radial direction of the embossing roll can be easily varied in a wide range if such angle is the same for all embossing elements.
Another technique predominantly used for rolls made of plastics material, like rubber rolls, is the use of a laser ablation technique. Laser techniques are used for rolls made of plastic material as well as for steel rolls. Other manufacturing methods for embossing rolls are manually engraving techniques or galvanoforming. In principle a combination of such techniques can be used for manufacturing embossing rolls. One preferred manufacturing process is based on masking/etching technology in combination with moletage (knurling). Another well-known combination of engraving techniques is a multi-step etching technique with at least five, preferably around 10 separate etching steps, and the final rounding of the resulting steps in order to achieve a smooth shape of the embossing elements produced. Useful rounding steps are brushing with metal brushes, short overall etching or blast treatment with hard particles, such as glass, sand or corundum.
The embossing not only serves to provide bulk to the fibrous product but also to provide an improved optical appearance to the product. The optical appearance of a product is important for consumer products and also serves to provide a higher degree of recognition to the product. The optical appearance can be improved by combining embossing and coloring steps. Another reason for embossing is to generate higher absorbency or improved perceived softness.